This invention relates to the determination of subcriticality of uranium bearing materials during nuclear fuel production. In particular, it relates to the automatic determination of subcriticality or moderator concentration in uranium dioxide powder by a subcriticality measurement apparatus and method. The apparatus includes cadmium thermal neutron shields to facilitate and insure the accuracy of the subcriticality measurements. Measuring the moderator concentration is important, since moderators are effective for slowing or thermalizing fission-produced fast neutrons, which enable the establishment of a nuclear chain reaction.
Typical uranium fuel material comprises the isotope U-238 enriched in the order of 1-4 percent with the isotope U-235. Typical uranium containing fuel elements are shown, for example, by Specker et al in copending application Ser. No. 952,846, filed Oct. 19, 1978, now U.S. Pat. No. 4,285,769.
Uranium is typically stored in the form of uranium dioxide powder in five-gallon steel containers during the process of manufacturing nuclear fuel. To avoid any risk of the combined mass of uranium at any given location becoming critical--that is, capable of sustaining a nuclear chain reaction--the containers are stored individually and separated from one another at a considerable administrative and economic expense.
One difficulty encountered in securing criticality safety for a large number of stored uranium containers is the uncertainty of knowing the subcriticality of any given one of the containers. If the subcriticality of each container were conveniently and accurately determinable, then one could simply calculate the subcriticality of the overall system of individual containers with precision. Unfortunately, no such convenient and accurate measurement technique has been available prior to this invention. This has hampered the development of efficient storage and stockpiling techniques for uranium.
Individual uranium containers of the type utilized in nuclear fuel production are substantially subcritical under all moderator conditions. Accordingly, it is useful to speak in terms of subcriticality rather than criticality. For purposes of this disclosure, we need not speak of degrees of criticality, since the object of criticality safety programs is to prevent any degree of criticality in any portion of the uranium destined for use as reactor fuel.
Many variables enter the subcriticality calculation, including overall volume, structure, uranium density, enrichment, and of most concern with respect to this invention, the moderator concentration, which primarily involves moisture, or the water content found in the uranium. Uranium density, enrichment, and container volume or structure are generally relatively fixed parameters. The subcriticality parameter of most concern, however, the one susceptible of the greatest variation, is the moderator concentration.
Whatever its origin, the concentration of water in a mass of uranium stored for nuclear fuel production may vary from zero to over 100,000 parts per million. As will be seen, in the context of managing the storage of a system of plurality of uranium containers, a moderator concentration of over 20,000 parts per million is unacceptable for a given container and storage array geometry, which may for example include uranium dioxide contained in five-gallon cans. Of course, an individual container taken independently and separated from other like containers can be flooded with water and still be completely subcritical. A single five-gallon can in which the uranium is frequently stored, for example, is small enough to be subcritical even when submerged in water.
Destructive chemical analysis of samples to determine the subcriticality of each uranium container in a plant or storage area is impractible, since each container must then be opened, and a quantity of uranium is lost in the process. Furthermore, the results of such a chemical test may not be available for hours or even days.
Humidity sensors to measure subcriticality are, as a practical matter, inoperative in the high moisture or moderator environment involved. Moreover, conventional humidity sensors are typically effective to measure ambient humidity, but not the concentration of liquid water. Beyond this, many commercially available humidity sensors are simply unreasonably expensive.